Beads of ceramic material

ABSTRACT

The invention relates to beads used for crushing or dispersion which are formed by fusion, conversion into beads and solidification of an appropriate batch consisting essentially, on the oxide basis, of up to 85% by weight of ZrO 2 , a proportion of SiO 2  such that the weight ratio ZrO 2  /SiO 2  is greater than or equal to 1.5, an optional proportion of Al 2  O 3  such that the weight ratio Al 2  O 3  /SiO 2  is 0 to 1.5 and an optional proportion of Na 2  O such that the weight ratio Na 2  O/SiO 2  is from 0 to 0.04, the batch also containing at least one of the additional oxides of MgO and CaO in proportions such that the weight ratio MgO/SiO 2  is from 0.03 to 1 and the ratio CaO/SiO 2  is from 0.03 to 1.45.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to beads or small balls of ceramicmaterial for use inter alia in methods and devices for micro-crushingand for producing a dispersion in a moist medium.

2. Description of the Prior Art

In recent years there has been a spectacular development in devices andmethods for micro-crushing and for producing a dispersion in a moistmedium. The devices (bead-mills) and methods use dispersing or crushingagents which are spherical and have a small diameter (e.g. 0.4 to 4 mm)and are used, e.g., to disperse the constituents of paints, varnishes,inks or the like introduced in liquid or solid form, or to micronise apowder in a moist medium, the powder having been pre-crushed byconventional methods (e.g. in a ball-mill).

Briefly, dispersion or crushing agents must have the followingproperties:

Mechanical resistance to crushing and impacts;

Resistance to abrasion; and

A particle size suitable for the application.

In addition, a high density and the property of not coloring theprocessed products are important in certain applications, e.g., in thetreatment of slips or very viscous mixtures in the first case and thetreatment of white paints or powders in the second case.

The original dispersing agent used in these devices and methods wassand, e.g., Ottawa sand having round grains. If this natural product isused, however, the devices are not as efficient as they might be, sinceit is available only within a limited range of particle sizes(0.4-0.8mm) and its resistance to abrasion is relatively low, whichlimits its use to the treatment of relatively soft powder. Attempts havetherefore been made to develop dispersing or crushing agents which aremore suited to requirements than sand is. For example, glass beads andsintered ceramic beads are at present commercially available.

Glass beads are available within a wide range of diameters and givesatisfactory results when the substances processed are not veryabrasive. However, they have little resistance to abrasion and haverelatively low density (approx. 2.6), and are therefore unsuitable forprocessing excessively viscous or abrasive products. They also have lowresistance to impact and are progressively destroyed by flaking or evensplitting. New beads have good resistance to crushing, but during usethis property is rapidly lost as a result of microscopic fissures,scratches or surface splitting.

Beads of ceramics sintered at high temperature, more particularlytitanium dioxide or alumina, have better resistance to abrasion thanglass beads, but the resistance is still limited by the quality ofsintering, i.e., the quality of the intercrystalline bonds. The methodof manufacturing these sintered beads is still secret.

There is thus a need for beads having further improved properties.

U.S. Pat. No. 2,924,533 discloses spherical particles made ofcrystalline zirconium dioxide with or without crystalline mulliteembedded in a vitreous silica-containing material containing, by weight,22.5-75% zirconium dioxide, 22.5-55% silica and 0-22.5% alumina andhaving good mechanical strength. These particles are described as of useas blasting agents (sanding).

The invention relates to higher-quality ceramic beads for use inter aliaas dispersion or crushing agents.

SUMMARY OF THE INVENTION

The invention relates to beads of ceramic material obtained by fusion,conversion into beads and solidification of appropriate batch consistingessentially, on the oxide basis, of up to 85% by weight of ZrO₂, aproportion of SiO₂ such that the weight ratio ZrO₂ /SiO₂ is greater thanor equal to 1.5, an optional proportion of Al₂ O₃ such that the weightratio Al₂ O₃ /SiO₂ is 0 to 1.5, and an optional proportion of Na₂ O suchthat the weight ratio Na₂ O/SiO₂ is from 0 to 0.04, the batch alsocontaining at least one of the additional oxides MgO and CaO inproportions such that the weight ratio MgO/SiO₂ is from 0.03 to 1 andthe ratio by weight CaO/SiO₂ is from 0.03 to 1.45.

Usually the ZrO₂ content is at least about 23%.

It has unexpectedly been found that the presence of at least one of theadditional oxides MgO and CaO considerably improves the properties ofbeads containing zirconium dioxide, silica and, optionally, alumina,compared with beads not containing such additional oxides, such as thebeads disclosed in U.S. Patent 2,924,533.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a ternary diagram of the system SiO₂ --MgO--CaO.

FIG. 2 is a ternary diagram of the system SiO₂ --MgO--Al₂ O₃.

FIG. 3 is a ternary diagram of the system SiO₂ --CaO--Al₂ O₃.

The expression "consisting essentially" should be understood as meaningthat ingredients other than these specified can be present in the batchas long as they do not materially affect the desirable properties of thefinal beads. Such unmentioned ingredients can be, in particular,impurities present in the raw materials forming the batch or ingredientswhich are volatilized during the melting of the batch.

The manufacture of beads according to the invention does not presentspecial difficulties. The starting batch, comprising the aforementionedoxides or precursors thereof, can be melted in an electric furnace orother fusion device well-known to the skilled artisan. The moltenmaterial can be converted into beads by blowing a jet of molten material(e.g. with air or steam) and thus dispersing it into a number ofparticles which assume spherical form owing to viscosity and surfacetension. Methods of this kind are conventionally used for manufacturingcommercial glass beads (see e.g. U.S. Pat. No. 3,499,745). Beads havinga diameter between a few tenths of a mm and 4 mm can be produced in thisway.

After cooling, the spherical particles or beads according to theinvention are made up of round, non-overlapping crystals of zirconiumdioxide embedded in a vitreous material formed from silica and the MgO,CaO, Al₂ O₃ and Na₂ O present.

The beads according to the invention are substantially solid (free fromcentral cavities and microscopic fissures) and very resistant toabrasion and crushing, owing to the hardness of the constituent phases(zirconium dioxide and silica glass improved by additions) and theexcellent cohesion due to the glass, which very efficiently "wets" thezirconium dioxide crystals. The beads have been found very useful interalia in dispersion and micro-crushing operations.

Before studying the influence of optional and additional oxides on theproperties of beads according to the invention we shall set out the testmethods used during this research.

Examination of polished cross-sections

A few grams of beads are embedded in a thermo-setting resin. Theresulting small test-piece is milled along a plane so as to obtain beadcross-sections. This treatment is sufficient to observe the porosity,shrinkage pipes and cracks, if any. In order to observe the crystallinetexture, however, it is preferable to polish the cross-sections, e.g.with diamond paste.

Analysis of phases with X-rays

This study is very important for following the effect of addingadditional oxides, since the appearance of new crystalline phases mayexplain important variations in the bead characteristics.

Resistance to crushing

For each bead composition, 20 beads are selected for their sphericityand are individually given crushing tests between the two pistons of apress. For the purpose of comparison, the test is always made on beadshaving the same diameter, i.e. 2 mm. The crushing strength is theaverage of the 20 results obtained.

Tests under industrial conditions have shown that for most applicationsthe average resistance to crushing E must be greater than or equal to 60kg per 2-mm bead. With beads having a crushing resistance below thisvalue, breakage is considerable and the beads are practically useless.

Resistance to abrasion

The resistance to abrasion is determined on a cast plate having the samecomposition as the beads. A test is made on the skin (external surface)and the cast plate is sufficiently thin (6 mm) for its texture to besubstantially identical with that of the beads.

A metal mask about 30 mm in diameter bounds the abraded surface inconstant manner. 1 kg of brown corundum (particle size 0.4-0.6 mm) issprayed from a sanding gum at an angle of 450 with respect to the plate,which is placed at 150 mm from the gun nozzle orifice. The pressure ofthe air supplying the gun is 2.5 kg/cm².

The plate is weighed before and after the test and the volume abraded(ΔV) is calculated.

An identical test is made on a standard plate made of a commercialrefractory electro-melted material (ZrO₂ = 32%, SiO₂ = 16%, Al₂ O₃ = 51%and Na₂ O = 1% by weight; E.R. 1681 material sold by the applicant),which is arbitrarily given the index 100. This standard material, whichhas long been used for its resistance to abrasion, is not part of theinvention.

When bead are prepared from this material, their structure is poor (i.e.overlapping crystals, microscopic pipes and microscopic cracks), andtheir resistance to crushing is too low (less than 60 kg). The abrasionresistance I of the tested material is calculated from the formula:##EQU1##

Hereinafter, all the percentages are by weight.

Beads made of SiO₂ and ZrO₂ alone (not according to the invention)

In the case of SiO₂ < 10%, melting in an arc furnace does not raise anyinsurmountable problem, but the casting temperature is very high (>2500°C) and it is difficult to maintain a uniform liquid jet, which is aprerequisite for good granulation. The casting lip frequently becomesfrequently plugged and the granules are very irregular.

In the case of SiO₂ > 50%, melting becomes increasingly difficult, sincethe increase in the silica content is accompanied by a decrease in theelectric conductivity of the bath and progressively higher operatingtemperatures are required. During casting, the product is particularlyviscous and tends to form fibres. The granulation output is very poor.

In the case of 10 ≦ SiO₂ ≦ 50%, the melting and granulating conditionsare acceptable, but of course are better in proportion to the distancefrom the limits.

Characteristics of beads obtained by fusion and granulation of ZrO₂-SiO₂ compositions with 10% ≦ SiO₂ ≦ 50%:

Density: the density increases in linear manner from 3.10 for SiO₂ = 50%to 4.8 for SiO₂ = 10%.

Examination of polished cross-section

10 < SiO₂ < 15%: Slight tendency to a residual central shrinkage pipeand the presence of microscopic cracks

15 ≦ SiO₂ ≦ 40%: Solid beads of very good appearance. Regular texture ofrounded ZrO₂ crystals in a SiO₂ glass

SiO₂ > 40% : Texture becoming increasingly heterogeneous with thecontent of SiO₂ : stratification of large areas of silica glass withmicroscopic fissures or cleavages.

Resistance to crushing

The following Table gives the resistance to crushing for different SiO₂contents.

    ______________________________________                                        SiO.sub.2, %       E                                                          ______________________________________                                        10              40 kg/2 mm-diameter bead                                      15              60 Kg/2 mm-diameter bead                                      20              65 kg/2 mm-diameter bead                                      30              80 kg/2 mm-diameter bead                                      40              90 kg/2 mm-diameter bead                                      50              60 kg/2 mm-diameter bead                                      ______________________________________                                    

Thus, the crushing strength is good in the cases where SiO₂ ≧ 15%.

Resistance to abrasion

Through the entire range, the abrasion resistance index I decreases in alinear manner when the SiO₂ content increases, as shown by the followingtable:

    ______________________________________                                               SiO.sub.2, %   I                                                       ______________________________________                                               50             IO                                                             30             75                                                             I0             140                                                     ______________________________________                                    

By way of example, the composition SiO₂ = 50%, ZrO₂ = 50% has anabrasion index comparable with that of glass.

All results show that useful compositions are obtained when theproportion by weight of SiO₂ is between 15 and 40% and the correspondingproportion of ZrO₂ is between 85 and 60% by weight, corresponding to aratio by weight ZrO₂ /SiO₂ of at least 1.5.

Note that the best resistance to crushing is obtained in the range 30% ≦SiO₂ ≦ 40% and the abrasion resistance increases with the ZrO₂ contentbut is very good through the entire range.

In short, the compositions in which

    ______________________________________                                        30% < SiO.sub.2 = 40%                                                                            (ZrO.sub.2 /SiO.sub.2 varying from 1.5 to 2.33)            60% ≦ ZrO.sub.2 ≦ 70%                                           ______________________________________                                    

are preferred owing to:

ease of manufacture,

compactness and absence of microscopic cracks

good resistance to crushing, and

good resistance to abrasion.

Note that all these compositions can be obtained from natural zirconsand (SiO₂.ZrO₂) containing approx. ZrO₂ = 66%, SiO₂ = 33% (+impurities). The use of zircon sand as a raw material in the manufactureof beads according to the invention is very advantageous economicallyand constitutes a preferred embodiment.

In the case where SiO₂ is required to be > 33%, it is sufficient to addthe appropriate quantity of silica sand to the batch to be melted.

In the case where SiO₂ is required to be < 33%, it is known that ifzircon is melted under reducing conditions (e.g. by adding wood carbon),some or all of the silica can be eliminated by the reaction: ##EQU2##

By operating under controlled conditions, therefore, the proportion ofsilica can be reduced. This method is much more economic than addingZrO₂, which is very expensive.

The effect of optional oxides and additional oxides

The effect of optional oxides and additional oxides was studied on abasic composition containing approx. 33% SiO₂ and approx. 66% of ZrO₂(ZrO₂ /SiO₂ = 2), i.e. natural zircon sand, because the last-mentionedsubstance is the raw material supplying the cheapest zirconium dioxide.The proportion of optional oxides and additional oxides are shown in theform of the weight ratio: ##EQU3##

The proportions of optional or additional oxides ##EQU4## indicated inthe case of zircon sand are also valid for the other compositionsaccording to the invention, since these oxides only modify the nature ofthe vitreous material.

The effect of alkali-metal oxides

The addition of alkali-metal oxides results in greater fluidity of themolten mixture. The beads are solid but increasingly small. The castingtemperatures are lower but still high (2000° C for an addition of 10% byweight).

We mainly studied the effects of adding Na₂ O, which is the mostimportant substance economically. The other alkali-metal oxide (K₂ O andparticularly Li₂ O) are expensive and their use is not justified unlessthe improvement is spectacular. This is not the case, however, as shownby results obtained with Na₂ O and sampling with K₂ O and Li₂ O.

It is also known that alkali-metal fluxes associated with silica giveglass which easily deteriorates and more particularly has a poorresistance to water and to atmospheric and chemical agents. Clearly,this glass will deteriorate even more quickly if abraded.

The addition of alkali-metal substance does not improve the beadcharacteristics. The resistance to crushing, impact and abrasiondecreases and becomes unacceptable at a weight ratio Na₂ O/SiO₂ > 0.2.

Small additions corresponding to a ratio Na₂ O/SiO₂ ≦ 0.04 have only aslight influence on the caracteristics but appreciably facilitatemelting of the most siliceous product.

Note that losses of Na₂ O through volatilisation are very importantduring fusion, so that the Na₂ O content in the produced beads can besmaller than in the starting batch.

The effect of Al₂ O₃

The range 0 < Al₂ O₃ /SiO₂ < 2.7 was scanned.

The external appearance of the beads is very good for all thesecompositions. The polished surface does not show any marked tendency toresidual shrinkage pipes or cracks. X-ray analysis shows that monocliniczirconium dioxide is the only crystalline phase. However, mullite linesappear in the case where Al₂ O₃ /SiO₂ > 1.5.

Resistance to crushing

This increases very rapidly and reaches 100 kg after the addition of asmall amount of alumina corresponding to the ratio Al₂ O₃ /SiO₂ = 0.1,and then remains almost constant up to Al₂ O₃ /SiO₂ = 0.6. It thenslowly decreases but remains greater than 80 kg up to Al₂ O₃ /SiO₂ = 1.Beyond this, it decreases to below 60 kg for Al₂ O₃ /SiO₂ = 1.5.

Abrasion index

This increases up to Al₂ O₃ /SiO₂ = 1.5 (I = 130), then decreases: I =80 for Al₂ O₃ /SiO₂ = 2.7.

In conclusion, the addition of alumina to SiO₂ -ZrO₂ mixtures improvethe bead characteristics for Al₂ O₃ /SiO₂ ≦ 1.5.

The best characteristics are obtained for 0.1 < Al₂ O₃ /SiO₂ < 1.

E = 100 kg/2-mm diameter bead

I = 100

effect of MgO

The range 0.03 < MgO/SiO₂ < 1.86 was scanned. Throughout this range,regular beads were obtained having a good external appearance.

In the case of MgO/SiO₂ ≦ 1, examination of polished surfaces shows thatthe beads are solid, without cracks and with a very fine texture.

X-rays analysis shows that the main phase is monoclinic zirconiumdioxide with a small amount of cubic zirconium dioxide. The magnesiumsilicate is amorphous.

In the case where MgO/SiO₂ > 1, a central shrinkage pipe appears andincreases with the ratio MgO/SiO₂. This fault is apparently connectedwith the formation of forsterite (2 MgO.SiO₂) which precipitates at hightemperature. This compound has been detected by radiocrystallographicanalysis and its concentration appears to be related with the size ofthe shrinkage pipe.

Resistance to crushing

The resistance to crushing increases with the weight ratio MgO/SiO₂. Itpasses through a maximum for MgO/SiO₂ = 0.4, then decreases.

In the case where MgO/SiO₂ ≦ 0.77, E > 80 kg/2 mm diameter bead, i.e.the same as for zircon beads without addition.

In the case where MgO/SiO₂ > 1, this characteristic becomes unacceptable(< 60 kg per 2-mm diameter bead).

Abrasion index

The abrasion index increases with MgO/SiO₂, reaching a maximum of 150when the ratio is 0.4. It then decreases, but remains better than forcompositions without additions, up to MgO/SiO₂ = 1. In short, theaddition of MgO to SiO₂ -ZrO₂ mixtures improves the bead characteristicsin the case where Mgo/SiO₂ ≦ 1.

The best characteristics are obtained when MgO/SiO₂ is approximately0.4, i.e.

E = 145 kg/2-mm diameter bead, and

I = 150.

effect of CaO

The range 0.03 < CaO/SiO₂ < 1.90 was scanned.

Throughout this range, the beads obtained were regular and had a goodexternal appearance.

Examination of polished cross-sections showed that the beads were solid,without cracks and contained fine zirconium dioxide crystals up toCaO/SiO₂ = 1.5.

Below this value, shrinkage pipes begin to appear and increase when theratio increases.

X-ray analysis shows that this fault corresponds to the presence ofCaO.ZrO₂ and crystalline silicates.

These results were confirmed by studying the characteristics of theresulting beads.

Resistance to crushing

This increases with the CaO/SiO₂ ratio, passing through a maximum (120kg/bead) for CaO/SiO₂ = 0.82. It then decreases and becomes unacceptable(< 60 kg) for CaO/SiO₂ = 1.45. In the case where CaO/SiO₂ ≦ 1.21, E > 80kg (the resistance of zircon beads without additions).

Abrasion index

It increases up to CaO/SiO₂ = 1.1 (I = 124) then begins to decrease, butremains greater than 75 for a CaO/SiO₂ ratio ≦ 1.45.

In conclusion, the addition of CaO to molten mixtures of SiO₂ and ZrO₂improves the bead characteristics for CaO/SiO₂ = 1.45.

The best results are obtained for a weight ratio CaO/SiO₂ ˜ 0.82, i.e.

E = 120 kg/2-mm diameter bead, and

I = 120.

the effect of adding two or more optional and additional oxides

As before, the effect was studied by adding optional and/or additionaloxides to a basic composition comprising natural zircon sand (33% SiO₂and 66% zirconium dioxide approximately).

Since the optional and additional oxides only modify the vitreousmaterial, the results obtained with the systems SiO₂ -- MgO -- CaO, SiO₂-- MgO -- Al₂ O₃ and SiO₂ -- CaO -- Al₂ O₃ have been shown in theaccompanying FIGS. 1, 2 and 3, which are ternary diagrams of thesesystems, which make up the vitreous phase of the beads according to theinvention.

The important regions in these diagrams are to the left of thecontinuous curve. The important regions of the systems SiO₂ -- MgO --Al₂ O₃ and SiO₂ -- CaO -- Al₂ O₃ are superposable on their most part, sothat the conclusions remain valid for the system SiO₂ -- Al₂ O₃ --(CaO + MgO) within the same limits, i.e. for a given composition a partof the CaO or MgO can be replaced by MgO or CaO, respectively. Thesediagrams also show the results obtained with compositions containingonly one optional or additional oxide (points on the sides of theternary-diagram triangle). On the diagrams, the first and the secondFigures associated with each indicated composition denote the resistanceto crushing and the resistance to abrasion thereof, respectively.

The additions to zircon sand (33% ZrO₂ -66% zirconium dioxide) result inan increase in the proportion of the vitreous binding matrix, which isthe least hard phase in the beads according to the invention. In spiteof this, there is a very marked improvement in the resistance tocrushing and abrasion. This is due to the improvement in the mechanicalcharacteristics of the thus-formed vitreous matrices.

Similarly, compositions having a higher content of zirconium dioxidebonded by the best vitreous matrices disclosed hereinbefore, will havegreatly improved characteristics, more particularly a very highresistance to abrasion. We shall now give some examples of compositionshaving a high zirconium dioxide content modified by additional oxideswhich are useful in manufacturing bead according to the invention, withtheir crushing and abrasion resistances.

    ______________________________________                                                         ZrO.sub.2 = 79% by weight                                                                       E = 150                                    Composition A    SiO.sub.2 = 15% by weight                                                                       I = 200                                                     MgO = 6% by weight                                                            ZrO.sub.2 = 76% by weight                                                                       E = 130                                    Composition B    SiO.sub.2 = 12% by weight                                                                       I = 160                                                     CaO = 12% by weight                                                           ZrO.sub.2 = 74% by weight                                                                       E = 150                                                     SiO.sub.2 = 7% by weight                                     Composition C    Al.sub.2 O.sub.3 = 10% by weight                                              CaO = 6% by weight                                                                              I = 300                                                     MgO = 3% by weight                                           ______________________________________                                    

We claim:
 1. Beads of ceramic material having a diameter of betweenabout a few tenths of a mm and about 4 mm obtained by fusion, conversioninto beads and solidification of a starting batch consisting, on theoxide basis, of up to 85% by weight of ZrO₂, a proportion of SiO₂ suchthat the weight ratio ZrO₂ /SiO₂ is greater than or equal to 1.5, anoptional proportion of Al₂ O₃ such that the weight ratio Al₂ O₃ /SiO₂ is0 to 1.5 and an optional proportion of Na₂ O such that the weight ratioNa₂ O/SiO₂ is from 0 to 0.04, the starting batch also containing atleast one of the additional oxides MgO and CaO in proportions such thatthe weight ratio MgO/SiO₂ is from 0.03 to 1 and the weight ratioCaO/SiO₂ is from 0.03 to 1.45.
 2. Beads according to claim 1, whereinthe weight ratio ZrO₂ /SiO₂ is between 1.5 and 2.33.
 3. Beads accordingto claim 2, wherein the weight ratio MgO/SiO₂ is between 0.03 and 0.77,the weight ratio CaO/SiO₂ is between 0.03 and 1.21, the weight ratio Al₂O₃ /SiO₂ is between 0 and 1 and the weight ratio Na₂ O/SiO₂ is between 0and 0.04.
 4. Beads according to claim 2, wherein the weight ratio ZrO₂/SiO₂ is equal to approximately
 2. 5. Beads according to claim 2,wherein the weight ratio MgO/SiO₂ is approximately 0.4.
 6. Beadsaccording to claim 2, wherein the weight ratio CaO/SiO₂ is approximately0.82.
 7. Beads according to claim 2, wherein the weight ratio Al₂ O₃/SiO₂ is between 0.1 and 1.